1. Field of the Invention
The present invention relates to a fuel cell system and a fuel cell control method. More specifically, it relates to a vehicle-installed fuel cell system and a fuel cell control method.
2. Description of Related Art
Recently, fuel cell systems have drawn attention as new sources of power that can be used to drive vehicles. For example, a fuel cell system can be configured with a fuel cell which produces electric power from chemical reactions of reactive gases, wherein a reactive gas supplying unit which supplies reactive gases to the fuel cell via a reactive gas channel, and a control unit controls the reactive gas supplying unit.
For example, the fuel cell may have a stacked structure of several tens or hundreds of stacked cells. Here, each cell is configured with a membrane electrode assembly (MEA) sandwiched between a pair of separators. The membrane electrode assembly is configured with two electrodes: an anode (positive electrode) and a cathode (negative electrode), and a solid polymer electrolyte membrane which is sandwiched between those electrodes.
The supply of hydrogen gas and oxygenated air as reactive gases to the anode and cathode of the fuel cell, respectively, causes an electrochemical reaction, from which the fuel cell produces electric power. In this case, since only basically harmless water is generated during power production, the fuel cell has attracted attention from the viewpoint of environmental impact and availability.
With the aforementioned fuel cell system, the total power production is controlled depending on the state of the fuel cell in order to reduce the load of the fuel cell. For example, a fuel cell system, which determines whether cell voltages are stable or unstable according to variations in water temperature, gas temperature, and cell voltage of the fuel cell, and controls the total power production by the fuel cell depending on the respective states using an electric current control threshold map when stable or unstable, has been proposed in the Japanese Unexamined Patent Application Publication No. 2005-183126 (hereafter, referred to as JP'126).
The fuel cell system disclosed in JP'126 controls the power producing current of the fuel cell depending on the stable or unstable states, allowing an improvement of the current characteristics in unstable conditions.
When moisture on the MEA surface turns to ice, the area where the ice is formed becomes unavailable for power production, resulting in a decrease in the areas on the MEA surface available for power production. In this case, activating the fuel cell increases the current density locally in the areas available for power production to keep the cell voltages constant.
However, the fuel cell system of the Patent disclosed in JP'126 is configured so as to control the total power production by the fuel cell depending on variations in water temperature, gas temperature, and cell voltage of the fuel cell. Accordingly, the total power production by the fuel cell is not limited as long as the cell voltages can be kept constant even if the fuel cell system is activated below freezing. As a result, the partial current density of the membrane which forms the MEA may become excessively high, possibly accelerating the deterioration of the membrane in that area.